Self-electrooptical effect devices offer considerable promise in the field of optoelectronic processing. Because of the low energy requirements of quantum well components, such devices can receive optical inputs and provide optical outputs with a high degree of efficiency. Moreover they can be combined in simple circuits to perform useful functions, such as optical switching, with minimal conductive paths, reduced parasitic capacitance and few line termination problems. For a survey of such devices and their applications, see D. A. B. Miller, "Quantum Well Optoelectronic Switching Devices," International Journal of High Speed Electronics, Vol. 1, No. 1, pp. 19-46 (1990).
A conventional SEED device described in the above-cited Miller article comprises a pair of quantum well PIN diodes tonned on a quantum well reflector stack. The two diodes are connected in series to a voltage source and separate beams of light are shone through the respective diodes to the reflector stacks where the beams are reflected back. The output of either diode can be modulated by vanation in either the optical input to the other diode or by variation in the voltage to the node between the diodes. Thus the device provides both optical input and optical output.
One shortcoming of SEEDs for use in optoelectronic switching is the low contrast ratio in the reflected beam intensity. Even in the "off" condition the device will partially reflect an incident beam, and the contrast ratio between a reflected "off" beam and a reflected "on" beam is typically sufficiently low (approximately 3) that the state of the device must be read by differential techniques. The "on" beam and the "off" beam must be compared in order to determine the condition of the switching device. This differential approach requires twice the laser power and twice the number of signal processing components than would be required for direct reading.